Interpretive Summary: Fish in aquaculture production environments are often exposed to multiple stressors including biological, chemical and physical entities. Studies that aim to characterize stress responses typically focus on the impacts of single stressors, disregarding possible interactive effects arising from aggregate exposure to multiple stressors. We subjected rainbow trout to the parasite known to cause proliferative kidney disease and an estrogen compound found in aquatic environments. Using gene expression analyses we determined that the response to multiple stressors was not additive based on the responses to individual stressors, that responses to multiple stressors are unique, and that in a combined response one stressor may have a more pronounced effect.

Technical Abstract:
The aim of the present study was to examine the molecular and organism reaction of rainbow trout, Oncorhynchus mykiss, to the combined impact of two environmental stressors. The two stressors were the myxozoan parasite, Tetracapsuloides bryosalmonae, which is the etiological agent of proliferative kidney disease (PKD) and a natural stressor to salmonid populations, and 17 -estradiol (E2) as prototype of estrogen-active chemical stressors in the aquatic environment. Both stressors, the parasite and estrogenic contaminants, co-exist in Swiss rivers and are discussed as factors contributing to the decline of Swiss brown trout populations over the last decades. Using a microarray approach contrasting parasite-infected and non-infected rainbow trout at low or high estrogen levels, it was observed that molecular response patterns under joint exposure differed from those to the single stressors. More specifically, three major response patterns were present: (i) expression responses of gene transcripts to one stressor are weakened by the presence of the second stressor; (ii) expression responses of gene transcripts to one stressor are enhanced by the presence of the second stressor; (iii) expression responses of gene transcripts at joint treatment are dominated by one of the two stressors. Organism-level responses to concurrent E2 and parasite treatmentassessed through measuring parasite loads in the fish host and cumulative mortalities of trout were dominated by the pathogen, with no modulating influence of E2. The findings reveal function- and level-specific responses of rainbow trout to stressor combinations, which are only partly predictable from the response to the single stressors.